1. Filed of the Invention
The present invention relates mainly to a liquid crystal composition suitable for active matrix (AM) elements, and an AM element including the composition.
2. Related Art
Liquid crystal display elements are classified by liquid crystal operating modes into phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), and vertical alignment (VA) elements, and so forth. The liquid crystal display elements are classified also by driving modes into passive matrix (PM) and active matrix (AM) elements. The PM elements are further classified into static and multiplex elements, and so forth, and the AM elements are further classified into thin film transistor (TFT) and metal insulator metal (MIM) elements, and so forth. The TFTs are classified into amorphous silicons, polycrystal silicons, and continuous grain silicons, and the polycrystal silicons are further classified by production processes into high temperature type silicons and low temperature type silicons. Further, the liquid crystal display elements are classified based on light sources into reflection type elements utilizing a natural light, transmission type elements utilizing a backlight, and semi-transmission type elements utilizing both the natural light and the backlight.
These elements include a liquid crystal composition having suitable characteristics. The general characteristics of the composition should be improved to obtain an AM element having excellent general characteristics. Table 1 below summarizes relationships of the general characteristics between the composition and the AM element. The general characteristics of the composition will be explained further in terms of a commercially available AM element. The nematic phase temperature range of the composition relates to the temperature range in which the element can be used. The higher limit temperature of the nematic phase temperature range is preferably 70° C. or more, and the lower limit temperature is preferably −20° C. or less. The viscosity of the composition relates to the response time of the element. It is preferred that the element has a short response time to display a moving image. Accordingly, the composition preferably has a small viscosity, and more preferably has a small viscosity at a low temperature.
TABLE 1General characteristics of a liquidcrystal composition and an AM elementGeneral characteristics of aGeneral characteristics ofNocompositionan AM Element1Temperature range of aUsable temperature range isnematic phase is widewide2Viscosity is small1)Response time is short3Optical anisotropy isContrast ratio is largesuitable4Threshold voltage is lowElectric power consumptionis small and a contrastratio is large5Specific resistance is largeVoltage holding ratio issmall and a contrast ratiois large1)A liquid crystal composition can be injected into a cell in a short time.
The optical anisotropy of the composition relates to the contrast ratio of the element. An element of a VA or IPS mode, etc. utilizes an electrically controlled birefringence. Thus, to maximize the contrast ratio of the VA mode element, a product (Δn·d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the element is designed to be uniform. For example, the product is 0.30 to 0.35 μm in the VA mode and 0.20 to 0.30 μm in the IPS mode. The cell gap (d) is generally 3 to 6 μm, so that the optical anisotropy of the composition is generally within the range of 0.05 to 0.11. A large dielectric anisotropy of the composition contributes to a small driving voltage of the element. Thus, the composition preferably has a large dielectric anisotropy. A composition having a positive dielectric anisotropy is used for common AM elements, while a composition having a negative dielectric anisotropy is used for VA-mode AM elements. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the element. Accordingly, the composition preferably has a large specific resistance initially, and further preferably has a large specific resistance even after it has been used for a long time.
A composition having a negative dielectric anisotropy contains a compound having a negative dielectric anisotropy. For example, as such a compound, a 2-difluoromethyl-3-fluoro-1,4-phenylene-containing compound having a negative dielectric anisotropy is described in Patent Document 1, etc.    Patent Document 1: WO 2000-39063 A (U.S. Pat. No. 6,576,303 B1)